Newcastle disease viruses (NDV) represent novel viral vaccine vector candidates. These viruses, belonging to the group of negative strand RNA viruses, are natural pathogens of birds, and only in rare occasions infect humans, resulting in mild conjunctivitis. The absence of pre-existing immunity and its safety profile in humans makes this virus an attractive candidate for vaccine vector development. NDV is able to infect and express proteins in mammalian cells, but in most instances, viral replication is limited to only a few cycles of infection in non avian cells, resulting in limited production of viral progeny. We propose to conduct studies aimed towards demonstrating the potential use of NDV vectors expressing gag and env antigens in HIV vaccine strategies. For this purpose, we will explore different approaches to evaluate the impact of NDV replication and of NDV induction of innate immune responses in antigen presenting cells (dendritic cells) and on the inherent immunogenic properties of this virus, and devise potential improvements based on this evaluation. In addition, we will attempt to improve the immunogenicity of NDV by modulating its replication properties, tropism, cytokine profile expression, impact on host responses and antigen expression. The purpose of these studies will be 1) to enhance NDV immunogenicity while retaining attenuation in mammals, and 2) to determine the properties that make a viral vector an optimal immunogen. For the second goal, in collaboration with Drs. Schnell and Lyles and as part of this program project, we will compare NDV vectors to the also negative strand RNA virus vectors rabies viruses (RV) and vesicular stomatitis viruses (VSV). These three viruses have similar replication strategies, but different tropisms and very different impacts in host responses. The most promising NDV vectors will be investigated in prime/boost vaccine approaches in a monkey model of HIV infection. These studies will be conducted with the help of two centralized research cores, allowing for the generation of comparative safety and efficacy data between NDV, RV and VSV vectors, as well as the evaluation of the prime/boost vaccine approaches. RELEVANCE (See instructions): Very little is known about what properties makes a viral vector an optimal immunogen against HIV. Our studies will unravel several of these properties and at the same time, will develop improved NDV-HIV vectors as potential novel immunogens to be used in HIV vaccine approaches.